He’s been called both the world’s oldest teenager and one of the most generous philanthropists in the country. T. Denny Sanford, who started working at age 8, achieved enough personal and financial success in business to land him on the Forbes’ list of wealthiest Americans for several years. Now 75, for the past six years he’s been focusing that energy and work ethic to a different cause: health.

Sanford’s $400 million gift in 2007 to what is now called Sanford Health was the largest gift given to an American health care system (Sanford Health is a sponsor of WebMD). In addition to making Sanford the largest nonprofit health care system in the country, Sanford has his eye on a number of health issues that affect children.

One of those initiatives is The Sanford Project. Its goal is to find a cure for type 1 diabetes in Sanford’s lifetime. "You see these kids who have 15 pokes a day with a needle and the pain that they go through," Sanford says, "and you can identify with it. Diabetes is unfortunately one of those situations that we don't recognize because the people otherwise act normal. But that's only if they get the proper care and take the proper medications."

Sanford is also working with WebMD on a joint project to teach children about food and fitness, called Fit Kids. The site is aimed at children in three age groups and will help kids and teens learn about food, fitness, mood, and recharging. “Kids could have a very positive effect on their entire family,” says Sanford. “So that’s really what WebMD and Sanford Health can put together.”

Sanford’s interest in health was in part motivated by his family. At age 4, he lost his mother to breast cancer, and when he was 21, his father died of heart disease. "I think health is an area where you can make the biggest difference, and that's what I'm trying to do," he says.

Sanford Health is now building a network of Sanford Children's Clinics around the world in communities lacking pediatric care. Sanford is also excited about the idea of helping to cure type 2 diabetes, because it can be caused by environmental conditions (poor diet and not enough exercise) and so can be cured by environmental conditions (more exercise and better nutrition). "It is so much more easily cured," he says. "We can take care of that in a heartbeat."

Sanford says he initially got involved with children's health care "for the children. But when you meet pediatric staff in hospitals and at schools [you realize] it's not just a job for them, it's a passion to save lives and help these little tykes. Children do not have a voice and we're trying to give it to them."

His many gifts landed him on a different Forbes list in 2010: the nation’s most generous donors, where he ranked No. 3.